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1.
A nutcase. As in not right in the head. Always have their shirt tucked into their trousers which are half way up their chest.
A person who is most likely to come and regail you with stories in a pub of when they "Nutted this bloke" and how his helper is nicking money from him.
They vary in condition, some are worse than others, with the worst ones not having control of bladder. A common hobbie of a Jellyhead is shouting at passing cars.

Best avoided
"Look at that Jellyhead shouting at the traffic"
"Why's that Jellyhead walking about with his trousers round his ankles?"
by colujimbo November 08, 2006
 
2.
Colored or flavored condom.
John: Hey Lisa, your favorite color is purple right?
Lisa: Yeah, why?
John:Because I got some purple jellyheads we could use tonight
Lisa:Oooh really? I love the grape flavored ones!
by Gizzly Adamz September 17, 2009
 
3.
A Jealous Person, so jealous they have their own name.

Reasons to use: If someone is being overly jealous you have the right to let them know that they are being a jelly head.
Mitch: Hey I just won the power ball!!!

Johnny: Playing the lottery is stupid, I work for everything I do not collect hand outs

Mitch: You're such a Jelly Head haha yeaa im in it to win it.
by Mitch Brown August 22, 2011
 
4.
Stupid person,
normally a blond.
That girl i dipped last night was some JellyHead
by Arm Zox January 19, 2004
 
5.
A person dedicated to the music and vibes of the band Lemon Jelly. Usually found posting on jelly forums, grooving to jelly music or just smiling about life in general. A very fine group of individuals.
"My really cool friend is a total Jelly Head."
by Nervous Tension January 06, 2006